I re-read Will's article http://www.backpackinglight.com/cgi-bin/backpackinglight/frameless_backpacks_sotm_part2b_2011.html - good article
I tried to measure torso collapse on my pack - tricky measurement - and I am not unbiased
To simplify the problem, what you want is the shoulder straps to attach to pack at top perpindicular so no weight is carried vertically on shoulders:
And what you don't want is for them to attach at an angle, because then some weight will be carried vertically on shoulders:
Torso length is measured from some point on hips to the point where shoulder straps attach at the top. My simple measurement is to measure from the top of the waist belt. I think this is 1.5 inches above where you're supposed to, so I'll just add that.
My pack with 10 pounds. This is as little weight as I can do to measure unloaded case. I need some weight to expand the burrito. I measure 17.75 inch so torso length is 19.25 inches:
The masking tape/felt pen is at 17.5 inches.
19 pounds - this is my typical max weight. I measure 17.5 inches - torso length is 19 inches. Torso collapse is 19 / 19.25 = 98.75%. I've carried this comfortably for many miles on many trips:
26 pounds - this allows me to add a days worth of water for rare cases there's no water where I want to go. 17.25 inches - 18.75 inch torso length - collapse = 18.75 / 19.25 = 97.5%. I've haven't carried this much but maybe it's doable:
This is about the same as the best frameless packs in Will's paper. Jam weighs 29 ounces and carries 3000 in3. Mine weighs 16.5 ounces and maybe 2800 in3.
There are several problems to solve. First you need a solid waist belt that doesn't collapse. 1/8 inch foam, 200D fabric tube, 3D mesh against back, several rows of stitches, 3/4 inch webbing with buckle at front:
Next, firmly attach pack to waist belt. Several rows of zigzag 8 inches long. Through pack, 3D mesh, fabric, foam, fabric. At middle of waist belt, at top edge of waist belt.
Then you need the tent poles to compress the "burrito roll" pack into one solid package that doesn't collapse itself. Top view: